In-text Question (Page 18)
Question: You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer:
- Dip the red litmus paper into each test tube:
- If the red litmus paper turns blue, that test tube contains the base.
- If the red litmus paper remains red, that test tube contains either the acid or distilled water.
- Use the blue-turned litmus paper to test the remaining two test tubes (the ones where the red litmus remained red):
- If the blue-turned litmus paper turns red, that test tube contains the acid.
- If the blue-turned litmus paper remains blue, that test tube contains the distilled water.
In-text Questions (Page 22)
Question 1: Why should curd and sour substances not be kept in brass and copper vessels?
Answer:
Curd and other sour foods have acids in them. These acids can react with the metal of brass or copper vessels to form harmful substances. This can spoil the food and also cause health problems. That is why we should avoid keeping sour substances in brass or copper vessels.
Question 2: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Answer:
When an acid reacts with a metal, hydrogen gas is usually released.
Example: When zinc reacts with dilute sulphuric acid, it forms zinc sulphate and hydrogen gas.
Zn + H₂SO₄ → ZnSO₄ + H₂
To test for hydrogen gas, bring a burning matchstick near the mouth of the test tube. If the gas is hydrogen, it will burn with a ‘pop’ sound.
Question 3: Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Answer:
Metal compound A could be calcium carbonate (CaCO₃).
The reaction: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
The gas CO₂ puts out a burning candle.
In-text Questions (Page 24)
Question 1: Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer:
Acids like HCl and HNO₃ release H⁺ ions when mixed with water, making the solution acidic. Alcohol and glucose, even though they contain hydrogen, do not release H⁺ ions in water. Therefore, they are not acidic.
Question 2: Why does an aqueous solution of an acid conduct electricity?
Answer:
An acid in water releases H⁺ ions. These ions help carry electric current through the solution. Thus, an acidic solution conducts electricity.
Question 3: Why does dry HCl gas not change the colour of the dry litmus paper?
Answer:
Dry HCl gas does not produce H⁺ ions without water. The presence of water is needed to release H⁺ ions. Since dry litmus paper and dry HCl have no water, no ions are formed and no colour change occurs.
Question 4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer:
Dissolving acid in water releases a lot of heat. Adding acid to water slowly, with stirring, allows the heat to spread out safely. If you add water to a concentrated acid, a large amount of heat is released at once, which can cause the mixture to splash and harm you.
Question 5: How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer:
When you dilute an acid, you add more water. This decreases the number of H⁺ (H₃O⁺) ions per unit volume. So, the concentration of H₃O⁺ ions goes down.
Question 6: How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer:
Dissolving more base increases the number of OH⁻ ions per unit volume, so the concentration of OH⁻ ions increases.
In-text Questions (Page 29)
Question 1: You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Answer:
A solution with a lower pH has more hydrogen ions. So, solution A (pH 6) has more hydrogen ions than solution B (pH 8).
Solution A (pH 6) is slightly acidic, and solution B (pH 8) is slightly basic.
Question 2: What effect does the concentration of H⁺(aq) ions have on the nature of the solution?
Answer:
The more H⁺(aq) ions, the more acidic the solution. Fewer H⁺(aq) ions mean the solution is less acidic and possibly neutral or basic.
Question 3: Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
Answer:
Yes, basic solutions also have H⁺ ions, but they have many more OH⁻ ions. Because OH⁻ ions outnumber H⁺ ions, the solution is overall basic.
Question 4: Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Answer:
If the soil is too acidic, a farmer would add quick lime, slaked lime, or chalk to neutralize the acidity and bring the soil pH closer to neutral for better plant growth.
In-text Questions (Page 33)
Question 1: What is the common name of the compound CaOCl₂?
Answer:
The common name of CaOCl₂ is bleaching powder.
Question 2: Name the substance which on treatment with chlorine yields bleaching powder.
Answer:
Dry slaked lime [Ca(OH)₂] reacts with chlorine to produce bleaching powder.
Question 3: Name the sodium compound which is used for softening hard water.
Answer:
Washing soda (sodium carbonate, Na₂CO₃) is used to soften hard water.
Question 4: What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.
Answer:
On heating sodium hydrogencarbonate (baking soda), it forms sodium carbonate, water, and carbon dioxide.
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Question 5: Write an equation to show the reaction between Plaster of Paris and water.
Answer:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
(Plaster of Paris) + (Water) → (Gypsum)
Exercises (Page 34-35)
Question 1: A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Answer:
(d) 10
A solution that turns red litmus paper blue is basic, and a pH of 10 is basic.
Question 2: A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl (b) HCl (c) LiCl (d) KCl
Answer:
(b) HCl
Acid (HCl) reacts with egg-shells (calcium carbonate) to produce CO₂, which turns lime-water milky.
Question 3: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution required to neutralise it will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Answer:
(d) 16 mL
Doubling the volume of NaOH solution (from 10 mL to 20 mL) means you need double the volume of HCl solution (8 mL × 2 =16 mL) to neutralise it.
Question 4: Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer:
(c) Antacid
Antacids help neutralize excess acid in the stomach, relieving indigestion.
Question 5: Write word equations and then balanced equations for the reaction taking place when –
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
Answer:
(a) Word equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Balanced equation: Zn + H₂SO₄ → ZnSO₄ + H₂
(b) Word equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Balanced equation: Mg + 2HCl → MgCl₂ + H₂
(c) Word equation: Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
Balanced equation: 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
(d) Word equation: Iron + Hydrochloric acid → Iron chloride + Hydrogen
Balanced equation: Fe + 2HCl → FeCl₂ + H₂
Question 6: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Answer:
Activity:
• Take solutions of glucose and alcohol in beakers and dip electrodes connected to a bulb into them.
• The bulb does not glow.
• This is because they do not produce H⁺ ions in water. Without free ions, no current flows.
• Thus, even though glucose and alcohol have hydrogen, they do not behave like acids.
Question 7: Why does distilled water not conduct electricity, whereas rain water does?
Answer:
Distilled water is pure and has no ions. Rain water has dissolved gases and minerals from the air, which form ions. These ions help conduct electricity. That is why rain water conducts electricity but distilled water does not.
Question 8: Why do acids not show acidic behaviour in the absence of water?
Answer:
Acids release H⁺ ions only when they are dissolved in water. Without water, no H⁺ ions are formed, so acids do not show their acidic properties.
Question 9: Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4,1,11,7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Answer:
(a) pH 7 is neutral, so D is neutral.
(b) pH 11 is strongly alkaline, so C is strongly alkaline.
(c) pH 1 is strongly acidic, so B is strongly acidic.
(d) pH 4 is weakly acidic, so A is weakly acidic.
(e) pH 9 is weakly alkaline, so E is weakly alkaline.
Order of pH by increasing H⁺ concentration: 11 (C) < 9 (E) < 7 (D) < 4 (A) < 1 (B)
(Remember: Lower pH means higher H⁺ concentration.)
Question 10: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Answer:
Fizzing will be more vigorous in test tube A with HCl because HCl is a strong acid and produces more H⁺ ions. Acetic acid is weaker and produces fewer H⁺ ions, so it reacts more slowly with magnesium.
Question 11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer:
As milk turns into curd, it becomes more acidic. This will lower its pH below 6. The more sour it gets, the lower its pH becomes.
Question 12: A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Answer:
(a) Baking soda is basic, so it increases the pH slightly, making the milk less acidic.
(b) Since the milk is now less acidic, it takes longer for bacteria to make it sour enough to become curd, so setting of curd is delayed.
Question 13: Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:
Plaster of Paris reacts with moisture in the air and changes into hard gypsum. This spoils its property of being easily shaped. Keeping it in a moisture-proof container prevents it from becoming hard.
Question 14: What is a neutralisation reaction? Give two examples.
Answer:
A neutralisation reaction is when an acid and a base react to form a salt and water.
Examples:
- HCl + NaOH → NaCl + H₂O
- H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
Question 15: Give two important uses of washing soda and baking soda.
Answer:
Uses of Washing Soda (Na₂CO₃):
- Used as a cleaning agent for domestic purposes.
- Used in glass, soap, and paper industries and for softening hard water.
Uses of Baking Soda (NaHCO₃):
- Used in baking powder to make cakes and bread soft and fluffy.
- Used as an antacid to relieve acidity in the stomach.